diff --git a/lib/async/reactor.rb b/lib/async/reactor.rb
index 48016c54..609ff53f 100644
--- a/lib/async/reactor.rb
+++ b/lib/async/reactor.rb
@@ -23,6 +23,7 @@
 require_relative 'logger'
 require_relative 'task'
 require_relative 'wrapper'
+require_relative 'scheduler'
 
 require 'nio'
 require 'timers'
@@ -82,8 +83,56 @@ def initialize(parent = nil, selector: self.class.selector, logger: nil)
 			@ready = []
 			@running = []
 			
+			if Scheduler.supported?
+				@scheduler = Scheduler.new(self)
+			else
+				@scheduler = nil
+			end
+			
 			@interrupted = false
 			@guard = Mutex.new
+			@blocked = 0
+			@unblocked = []
+		end
+		
+		attr :scheduler
+		
+		# @reentrant Not thread safe.
+		def block(blocker, timeout)
+			fiber = Fiber.current
+			
+			if timeout
+				timer = self.after(timeout) do
+					if fiber.alive?
+						fiber.resume(false)
+					end
+				end
+			end
+			
+			begin
+				@blocked += 1
+				Fiber.yield
+			ensure
+				@blocked -= 1
+			end
+		ensure
+			timer&.cancel
+		end
+		
+		# @reentrant Thread safe.
+		def unblock(blocker, fiber)
+			@guard.synchronize do
+				@unblocked << fiber
+				@selector.wakeup
+			end
+		end
+		
+		def fiber(&block)
+			if @scheduler
+				Fiber.new(blocking: false, &block)
+			else
+				Fiber.new(&block)
+			end
 		end
 		
 		def logger
@@ -154,7 +203,7 @@ def yield(fiber = Fiber.current)
 		
 		def finished?
 			# TODO I'm not sure if checking `@running.empty?` is really required.
-			super && @ready.empty? && @running.empty?
+			super && @ready.empty? && @running.empty? && @blocked.zero?
 		end
 		
 		# Run one iteration of the event loop.
@@ -174,6 +223,18 @@ def run_once(timeout = nil)
 				@running.clear
 			end
 			
+			unless @blocked.zero?
+				unblocked = Array.new
+				
+				@guard.synchronize do
+					unblocked, @unblocked = @unblocked, unblocked
+				end
+				
+				while fiber = unblocked.pop
+					fiber.resume if fiber.alive?
+				end
+			end
+			
 			if @ready.empty?
 				interval = @timers.wait_interval
 			else
@@ -197,7 +258,7 @@ def run_once(timeout = nil)
 				interval = timeout
 			end
 			
-			# logger.debug(self) {"Selecting with #{@children&.size} children with interval = #{interval ? interval.round(2) : 'infinite'}..."}
+			# logger.info(self) {"Selecting with #{@children&.size} children with interval = #{interval ? interval.round(2) : 'infinite'}..."}
 			if monitors = @selector.select(interval)
 				monitors.each do |monitor|
 					monitor.value.resume
@@ -223,6 +284,8 @@ def run_once(timeout = nil)
 		def run(*arguments, **options, &block)
 			raise RuntimeError, 'Reactor has been closed' if @selector.nil?
 			
+			@scheduler&.set!
+			
 			initial_task = self.async(*arguments, **options, &block) if block_given?
 			
 			while self.run_once
@@ -231,6 +294,7 @@ def run(*arguments, **options, &block)
 			
 			return initial_task
 		ensure
+			@scheduler&.clear!
 			logger.debug(self) {"Exiting run-loop because #{$! ? $! : 'finished'}."}
 		end
 		
diff --git a/lib/async/scheduler.rb b/lib/async/scheduler.rb
new file mode 100644
index 00000000..94252fff
--- /dev/null
+++ b/lib/async/scheduler.rb
@@ -0,0 +1,102 @@
+# Copyright, 2020, by Samuel G. D. Williams. <http://www.codeotaku.com>
+# 
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+# 
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+# 
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+# THE SOFTWARE.
+
+require_relative 'clock'
+
+module Async
+	class Scheduler
+		if Fiber.respond_to?(:set_scheduler)
+			def self.supported?
+				true
+			end
+		else
+			def self.supported?
+				false
+			end
+		end
+		
+		def initialize(reactor)
+			@reactor = reactor
+			@wrappers = nil
+		end
+		
+		def set!
+			@wrappers = {}
+			Fiber.set_scheduler(self)
+		end
+		
+		def clear!
+			# Because these instances are created with `autoclose: false`, this does not close the underlying file descriptor:
+			# @ios&.each_value(&:close)
+			
+			@wrappers = nil
+			Fiber.set_scheduler(nil)
+		end
+		
+		private def from_io(io)
+			@wrappers[io] ||= Wrapper.new(io, @reactor)
+		end
+		
+		def io_wait(io, events, timeout = nil)
+			wrapper = from_io(io)
+			
+			if events == IO::READABLE
+				if wrapper.wait_readable(timeout)
+					return IO::READABLE
+				end
+			elsif events == IO::WRITABLE
+				if wrapper.wait_writable(timeout)
+					return IO::WRITABLE
+				end
+			else
+				if wrapper.wait_any(timeout)
+					return events
+				end
+			end
+			
+			return false
+		ensure
+			wrapper.reactor = nil
+		end
+		
+		def kernel_sleep(duration)
+			@reactor.sleep(duration)
+		end
+		
+		def block(blocker, timeout)
+			@reactor.block(blocker, timeout)
+		end
+		
+		def unblock(blocker, fiber)
+			@reactor.unblock(blocker, fiber)
+		end
+		
+		def close
+		end
+		
+		def fiber(&block)
+			task = Task.new(&block)
+			
+			task.resume
+			
+			return task.fiber
+		end
+	end
+end
diff --git a/spec/async/scheduler_spec.rb b/spec/async/scheduler_spec.rb
new file mode 100644
index 00000000..4d715210
--- /dev/null
+++ b/spec/async/scheduler_spec.rb
@@ -0,0 +1,110 @@
+# Copyright, 2020, by Samuel G. D. Williams. <http://www.codeotaku.com>
+# 
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+# 
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+# 
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+# THE SOFTWARE.
+
+require 'async/scheduler'
+require 'async/barrier'
+require 'net/http'
+
+RSpec.describe Async::Scheduler, if: Async::Scheduler.supported? do
+	include_context Async::RSpec::Reactor
+	
+	it "can intercept sleep" do
+		expect(reactor).to receive(:sleep).with(0.001)
+		
+		sleep(0.001)
+	end
+	
+	describe 'IO.pipe' do
+		let(:message) {"Helloooooo World!"}
+		
+		it "can send message via pipe" do
+			input, output = IO.pipe
+			
+			reactor.async do
+				sleep(0.001)
+				
+				message.each_char do |character|
+					output.write(character)
+				end
+				
+				output.close
+			end
+			
+			expect(input.read).to be == message
+			
+			input.close
+		end
+		
+		it "can fetch website using Net::HTTP" do
+			barrier = Async::Barrier.new
+			events = []
+			
+			3.times do |i|
+				barrier.async do
+					events << i
+					response = Net::HTTP.get(URI "https://www.codeotaku.com/index")
+					expect(response).to_not be_nil
+					events << i
+				end
+			end
+			
+			barrier.wait
+			
+			# The requests all get started concurrently:
+			expect(events.first(3)).to be == [0, 1, 2]
+		end
+	end
+	
+	context "with thread" do
+		let(:queue) {Thread::Queue.new}
+		subject {Thread.new{queue.pop}}
+		
+		it "can join thread" do
+			waiting = 0
+			
+			3.times do
+				Async do
+					waiting += 1
+					subject.join
+					waiting -= 1
+				end
+			end
+			
+			expect(waiting).to be == 3
+			queue.close
+		end
+	end
+	
+	context "with queue" do
+		subject {Thread::Queue.new}
+		let(:item) {"Hello World"}
+		
+		it "can pass items between thread and fiber" do
+			Async do
+				expect(subject.pop).to be == item
+			end
+			
+			Thread.new do
+				expect(Fiber).to be_blocking
+				subject.push(item)
+			end.join
+		end
+	end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index d718307e..677bd84a 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
-require "covered/rspec"
+require 'async/rspec'
+require 'covered/rspec'
 
 if RUBY_PLATFORM =~ /darwin/
 	Q = 20